The invention relates generally to a device for inspecting a three-dimensional structure and more particularly to a device for inspecting three-dimensional printed circuit boards. The invention additionally relates to a surface structure inspection method.
A conventional device for inspecting a three-dimensional surface structure is known from German Patent Specification DE 196 08 468 C2. The device described in this German patent specification is suitable for inspecting a three-dimensional surface structure of a substantially flat test piece. In particular, the device is directed to inspecting the solder paste printing on printed circuit boards. Using an optical sensor, a partial area of the surface of the test piece is measured in three dimensions. A positioning device is used to position the optical sensor relative to the test piece, such that different partial areas of the surface are successively inspected.
One application in which the above-mentioned conventional device can be used is for inspection of the solder paste printing on printed circuit boards. To prevent potential solder defects from being transferred throughout the entire process chain in the production of electronic printed circuit boards, which would require subsequent repair of the boards at a substantial cost, the solder paste printing process must be constantly monitored. Monitoring the printing process enables the detection of defects caused by screen printing prior to the insertion of components on the printed circuit board and segregation of defective boards before additional costs are incurred.
Printed circuit boards for surface-mounted components, so-called surface mount technology (SMT) boards, are produced in large quantities and with many variations. The surface mounted components are fixed to the printed circuit board by soldering their terminals to xe2x80x9cmetallizedxe2x80x9d surfaces, referred to as pads, and are thereby also electrically connected with the printed conductors on the printed circuit board. For this purpose, a pattern of metallic pads corresponding to the position of the terminals of the components is provided on the printed circuit board. Solder paste is then deposited on the pads using a screen printing process. Thereafter, the surface mount component is mounted onto the printed circuit board. The component is initially held to the board by the adhesive property of the solder paste. Subsequently, after heating the printed board assembly, the terminals of the components are permanently soldered to the pads.
In the area of packaging technology, the trend is toward ever-increasing integration of the components with an increasing number of terminals per component housing. In the so-called fine-pitch range, the distance between two adjacent component terminals is approximately {fraction (1/40 )} of an inch. As a result, the pads on the printed circuit boards are also becoming smaller and more dense. About 80 percent of solder defects in the fine-pitch range are caused by solder paste printing. Examples of such defects are: insufficient solder paste deposit and short-circuits between adjacent pads due to inexact placement of the screen printing template during solder paste printing. To detect these defects, and to locate weak points in the production process, the printed circuit board is optically inspected after the solder paste has been deposited.
A second conventional device, described in German patent application number 199 15 052.4, comprises a device for inspecting a three-dimensional surface structure and a process for calibrating the device. This conventional inspection device is distinguished by improved accuracy in measuring three-dimensional surface structures. According to German patent application number 199 15 052.4, the coordinates of the characteristics to be inspected, particularly the solder deposit on printed circuit boards, can be derived from the mounting data of the components, which are normally available in the form of an electronic file after the design process of the components has been completed on a CAD design tool. One drawback to this conventional device, however, is that the user must manually calculate limit values for the geometric properties or the characteristics of the solder paste deposit and enter these values into the inspection device using a keyboard. In addition, the operator must manually enter the values for adjustments during any setup process. This work is very time-consuming and thus costly.
One object of the present invention is to provide a device and method for inspecting a three-dimensional surface structure in which the definition of limit values to which the measured values of the geometric properties of an inspected surface structure must conform is simplified for the operator.
To attain the above and other objects, a novel device and method in accordance with the present invention are proposed.
In accordance with one embodiment of the invention, a device is provided for inspecting a three-dimensional surface structure of a substantially flat test piece, the device includes an optical sensor operable to detect, in three-dimensions, at least a partial area of the surface of the test piece, a positioning device operable to position the optical sensor and the test piece relative to one another, a first memory operable to store setpoint values associated with geometric properties of the surface structure, a second memory operable to store tolerance values indicating a relative tolerance range for the geometric properties, an arithmetic logic unit operable to calculate limit values of an absolute tolerance range, and a display unit operable to display a defect if a measured value of at least one of the geometric properties of at least one of the inspected surface structures does not fall within a respective range of the absolute tolerance range.
In accordance with another embodiment of the invention, a surface structure inspection method is provided that includes generating a measured value by measuring a geometric property of a three-dimensional surface structure, calculating a limit value of an absolute tolerance range from a stored setpoint value for the geometric property and a stored relative tolerance value for the geometric property, comparing the measured value and the limit value, and generating a defect indication if the measured value fails to lie within the limit value.
In accordance with another embodiment of the invention, a device for inspecting the surface of a three-dimensional structure is provided which includes a first memory operable to store setpoint values for geometric properties of the three-dimensional structure, a second memory operable to store relative tolerance values corresponding to the setpoint values, and a logic unit operable to automatically calculate absolute tolerance values for the geometric properties, wherein the absolute tolerance values are based on respective values of the stored relative tolerance values and the stored setpoint values.
Also, a device and method in accordance with an embodiment of the present invention has the advantage that the process of defining the limit values, as discussed above, requires substantially less time than the time required for conventional devices. Accordingly, inspection of the solder paste printing on printed circuit boards in accordance with the present invention is less costly.
According to one embodiment, the limit values of the geometric properties of the solder paste deposit are calculated by a computer program that can be integrated into the control computer of the inspection device. As a result, the user of the inspection device is no longer required to perform time-consuming manual calculations.
A device and method in accordance with this embodiment of the present invention is advantageous particularly for inspecting the solder paste deposit on a printed circuit board. One reason this advantage is realized is because this type of inspection requires inspection of many different geometric properties and, thus, many different limit values need to be calculated.
According to one variant of the present embodiment, the operator can track the execution of the inspection program and the results of the individual inspection steps with an output device that outputs the measured values of the geometric properties of an inspected surface structure. For example, a display screen displays the measured values of the geometric properties and for those values that fall outside their absolute tolerance range, the values can be highlighted on the screen, particularly by their color, in contrast to the measured values of other geometric properties that fall within their absolute tolerance range. Accordingly, the operator""s attention is drawn directly to any possible defects.
According to a further embodiment, automatic correction of the limit values is advantageously achieved by (i) an input unit that the operator can use to specify whether a measured value of a geometric property that falls outside the absolute tolerance range should be evaluated as a defect and by (ii) an arithmetic logic unit that will adapt the absolute tolerance range of the geometric property according to the measured value if that value is not evaluated as a defect. This correction ensures ready adaptability of the inspection to any changes in the parameters of a production process. The operator is relieved of the manual entry of many numbers during the setup process. Also, the new limit values can be used as the basis of subsequent inspection steps.
To improve the decision basis for the subjective evaluation by the operator, a height image of the surface structure to be inspected may be recorded with the optical sensor and displayed on a screen. The height image is characterized by a particularly useful graphical appearance on the screen. The operator thus has access to all the information contained in the height image. As a result, the operator has a better decision basis than he or she would have had if the decision were based solely on the values of the optical properties of the surface structure to be inspected as measured by other image analysis processes.